1. Field of the Invention
The present invention relates to a method and an apparatus for three-dimensional holographic display suitable for video image display, using optical interference fringes recorded inside a photorefractive crystal which can display three-dimensional video images by irradiating a reference light thereon.
2. Description of the Background Art
The known methods for three-dimensional display of an object include a method based on stereography and a method based on holography. The former method based on stereography is a method in which images to be observed by left and right eyes are separately projected onto eyes, and only an observer can see a display target object three-dimensionally. The latter method based on holography is a method that faithfully reproduces wavefronts of lights reflected from an object.
The major factors that enable a human being to sense an object three-dimensionally include a binocular parallax and a focus adjustment. The method based on stereography is a method that reproduces only the binocular parallax, where the focus of the eyes is always set on a screen on which images are projected, so that there has been a problem that the eyestrain is inevitably caused by an observation over an extended period of time because the focal position is fixed despite of the object projection effect due to the binocular parallax.
On the other hand, the method based on holography is the most ideal method for three-dimensional display, in which both the binocular parallax and the focus adjustment can be realized exactly as in the real observation by the eyes. However, in order to record holograms, it is necessary to reproduce the light diffraction phenomenon, so that there is a need to record as many as several thousands of interference fringes per millimeter and reproduce a display from them. For this reason, the presently available holograms are all three-dimensional display of still pictures.
As a method for displaying moving objects by holography, there has been a proposition of a method using a liquid crystal panel, but the resolution (spatial frequency, pixel pitch) that can be displayed by the liquid crystal panel is insufficient for the holographic display. Namely, because of the low resolution (wide pixel pitch) and the limitation of the gray scale values that can be displayed, this method is still insufficient to display complicated fringes such as those of the actual holograms.
For this reason, in the holographic display using such electronic devices, it is only possible to display a few, small objects (point light sources). Moreover, because of the coarse resolution, there has been a problem that a separation of a light that is transmitted directly through the liquid crystal panel and the diffracted light is difficult, and the observation field is narrow.